Specifications
Working with Virtual Clusters
Equalizer Installation and Administration Guide 83
for these headers in every request on the server end, you need to disable the once only flag for the
cluster.
The once only flag is enabled by default when adding an L7 cluster. In general, it is more efficient
to enable once only, but in situations where match rule evaluation is very important, or where any
of the above effects are undesirable, once only should be disabled.
Using Active Content Verification (ACV)
Active Content Verification (ACV) is a mechanism for checking the validity of a server. When you
enable ACV for a cluster, Equalizer requests data from each server in the cluster and verifies that
the returned data contains a character string that indicates that the data is valid. You can use ACV
with most network services that support a text-based request/response protocol, such as HTTP.
However, you cannot use ACV with UDP-based services.
Controlling Server Verification Information
Specifying an ACV probe string and an ACV response string is one way to control the information
that Equalizer uses to verify the servers. Equalizer uses the probe string to request data from each
server. To verify the server’s content, Equalizer searches the returned data for the response string.
Equalizer expects to receive a response within the number of seconds specified by the probe
timeout global parameter (default 10) when performing active content verification. Equalizer will
make a number of successive attempts to reach the server equal to the strikeout threshold cluster
parameter, separated by the number of seconds specified by the probe delay cluster parameter.
If there is no response or the response string does not appear in the first 1024 characters of the
response, the verification fails; once the number of failures equals the strikeout threshold,
Equalizer marks the server down and stops routing requests to that server.
If requests come in that contain cookies for a persistent connection to the down server, Equalizer
will attempt to route the packets to the server in the cookie, and when this fails Equalizer sends the
request to the next available server in the cluster (depending on the load balancing algorithm for the
cluster). For the client, this means that any connection-related data stored on the downed server
(such as a shopping cart) will be lost, and the client will need to restart any operations begun that
depend on that data.
How ACV works is best explained using an example. The HTTP protocol enables you to establish a
connection to a server, request a file, and read the result. Figure 42 illustrates the connection process
when a user requests a telnet connection to an HTTP server and requests an HTML page.
Figure 42 Retrieving content from a server via telnet.
> telnet www.myserver.com 80
Connected to www.myserver.com
> GET /index.html
<HTML>
<TITLE>Welcome to our Home Page</TITLE>
</HTML>
Connection closed by foreign host.
User requests connection to server.
Telnet indicates connection is established.
User sends request for HTML page.
Server responds with requested page.
Telnet indicates server connection closed.